Add foundation keyboard shortcuts and plugin API

[wesbillman]

Implemented by Brain on behalf of Wes.

Summary

• One foundation-owned in-app shortcut dispatcher; plugins inject shortcuts and register through the same binding contract. Existing Cordis contribution ownership controls readiness, disable/re-enable, replacement and disposal.
• Command+, opens real Settings navigation and focuses main; Command+= / Command++ / Command+- / Command+0 control text size. Non-Apple platforms use Control. Preserve local handlers, exact modifiers, IME/AltGraph, editors and dialogs (including open Shadow DOM).
• Host-owned device-local text sizing (80–200%, 10% steps) with visible Appearance controls, persistence and retry. Actual message/composer and shared typography scale; window zoom, rem-based layout spacing and icons do not. Existing color storage is unchanged.
• Type-only author exports, documented conflict/focus/lifetime policy, and an independent ready-to-install shortcut-counter example.

Originating Buzz channel: keyboard-shortcuts (54af1452-92a0-4d46-ac41-e5b758f76044).

Verification

At 57228d608ff77337dcd4f49ecb7dff5dc9f6e312:

• Whole-tree Biome, TypeScript, author declarations and production frontend build passed.

• 48 focused tests across shortcut, appearance, app composition and plugin runtime files passed.

• All 8 shortcut journeys passed in Chromium/WebKit against the production frontend and fixture broker (not a native app or real relay). At first implementation snapshot e35e78c, the 26 combined shortcut/appearance/settings/plugin browser journeys passed; final follow-up only changes Space validation and Shadow DOM modal guards plus regression coverage.

• Ten isolated mutation probes at e35e78c detected broken Settings selection/home/focus, each zoom action, message/composer typography, listener teardown and plugin readiness. Both follow-up regressions at 57228d6 fail when their fixes are reverted. Clean-control shortcuts rerun passed. Mutation copies were separate from the review worktree.

• Independent source review found Space validation and Shadow DOM modal handling; both addressed in the second commit and independently rechecked. No remaining blockers from that review.

• A standalone TypeScript consumer compiles against generated @buzz/author; its host-only registration call is rejected by the type contract.

• Wes reported the local worktree try “works great” on 2026-09-11 after the desktop/web handoff. This records human acceptance of that try, not an exhaustive native/platform matrix.

Deferred / not integration-ready

• Broader integration scan / native checks; kept as a draft rather than claiming full validation. No native, dependency, capability or CSP changes.
• Custom third-party fixed-size/Shadow DOM typography needs its own adapter; arbitrary plugin CSS is not forcibly rewritten.

Try from the feature worktree with the pinned bin/just desktop (coordinate the existing dev app/port first), or bin/pnpm exec vite --port 5188 --strictPort for an isolated browser session. No developer .env or credentials are included.